Physiology Chapter 5: Skeletal Muscle Reflexes

Physiology of Skeletal Muscle Reflex Responses

• Skeletal muscles are typically voluntary, controlled by higher brain regions.
• They can also contract unconsciously in response to stimuli.
• MD137: Principles of Physiology course covers skeletal muscle reflex responses, focusing on muscle spindle apparatus and Golgi tendon organs.
• The course is taught by Dr. K.McCullagh.
• The nervous system plays a crucial role in controlling movements.
• See Chapter 5 in Medical Physiology (Rhoades & Bell, 4th Edition) for more detailed information on nervous system control of movement.

Classification of Muscle Types

• Muscles are categorized by control mode (voluntary/involuntary), anatomic structure (skeletal, cardiac, visceral), and histological features (striated/smooth).
• Involuntary reflexes are a subset of involuntary muscle activity.

Motor Control System

• The motor control system includes several interconnected components, including the cerebral cortex, basal ganglia, thalamus, brainstem, cerebellum, and spinal cord.
• α (alpha) motor neurons are the final common pathway, connecting the central nervous system to skeletal muscles.
• Peripheral sensory input plays a vital role in motor control.

Local Control of Motor Neurons

• Local control systems adjust motor neuron activity based on sensory information from muscles, tendons, and joints, to fine-tune movements.

Muscle Sensory Organs

• Muscle spindle apparatus responds to muscle length.

• Muscles with finer control have more spindles (e.g., extraocular muscles).

• Stretching a muscle stretches the spindles.

• Golgi tendon organs respond to tension on a tendon.

Muscle Spindle Apparatus

• Contains intrafusal fibers, thin muscle cells.
• Two types: nuclear bag fibers and nuclear chain fibers
• Two types of sensory nerve cells (afferent nerves) surround the fibers:
  • Primary (type Ia) on nuclear bag and nuclear chain fibers
  • Secondary (type II) on nuclear chain fibers

Muscle Spindle (Mechanism of Action)

• Diagrams show the function/structure of the muscle spindle apparatus.
• Key diagrams display the workings of the muscle spindle during different types of contractions including: no alpha or gamma nerve, alpha nerve, and alpha and gamma nerve coactivation.
• Alpha-gamma coactivation maintains spindle sensitivity during muscle contractions.
• Type Ia endings report both velocity and length of muscle stretch.
• Type II endings report muscle length.

Monosynaptic Stretch Reflex

• The simplest reflex; only a sensory neuron synapses with a motor neuron in the spinal cord.
• Maintains optimal resting length of skeletal muscles.
• Can be stimulated by striking the patellar ligament (knee-jerk reflex).

Knee-Jerk Reflex

• Sensory neuron activates an alpha motoneuron.
• Spindle stretches, activating sensory neuron.
• Alpha motoneuron stimulates extrafusal muscle fibers to contract.
• Stretch on the spindle is reduced.
• Striking the patellar ligament stretches the tendon, triggering the reflex.

Stretch Reflex (Protective Mechanism)

• Stretch reflexes protect muscles from damage.
• An added load stretches muscles, activating spindles that fire more.
• Muscle contracts to restore position, preventing over-stretching.
• Afferent sensory signals increase, resulting in spinal cord signals to muscles, increasing muscle activity.
• The process involves negative feedback.

Golgi Tendon Organs (GTOs)

• Constantly monitoring tension in tendons.
• Sensory neurons stimulate interneurons in the spinal cord.
• Interneurons inhibit motor neurons.
• This reduces tendon tension.
• A disynaptic reflex which involves two synapses.
• GTOs protect muscles and surrounding tissue from injury due to sudden movement or excessive load during activities such as athletic training.
  • Disinhibition is the process of minimizing GTO influence during training.

Flexor Withdrawal Reflex

• Painful stimuli on skin activate flexor muscles and inhibit extensor muscles on the same side of the body.
• This moves the affected limb away from the stimulus.
• The same stimulus produces an opposing response on the opposite side of the body.
• Reciprocal inhibition occurs, preventing conflict between muscles on different sides of the body.

Explore the physiology of skeletal muscle reflex responses in Chapter 5 of Medical Physiology, focusing on mechanisms such as the muscle spindle apparatus and Golgi tendon organs. This quiz delves into voluntary and involuntary muscle activity, as well as the role of the nervous system in motor control.